This invention relates to an electrolytic post-treating method of electrolytically chromate treated or metallic chromium plated steel sheet.
It is generally known that a film formed on the surface of an electrolytically chromate treated steel sheet comprises a metallic chromium layer and a hydrated chromium oxide layer, and no rust occurs on the said steel sheet surface under the effect of these two layers. There is a correlation between the said two layers formed on the surface of the said known electrolytically chromate treated steel sheet. An insufficient metallic chromium layer (one which is too thin) from among the said two layers results in the occurrence of many pores in the said metallic chromium layer. In order to maintain the level of corrosion resistance of the said steel sheet in this case, it is necessary to increase the thickness of the other hydrated chromium oxide layer. However, there is a limitation in the thickness of the said hydrated chromium oxide layer: a layer which is too thick impairs the surface quality of the said electrolytically chromate treated steel sheet with respect to deterioration of paint adhesion, formability and appearance.
With a view to avoiding these drawbacks, it has been proposed to reduce the thickness of the said hydrated chromium oxide layer and to increase that of the said metallic chromium layer. In this method, however, it is recognized that it it impossible to raise the production line speed and the formability worsens.
Some typical methods from among many methods conventionally available for the electrolytic chromate treatment of a steel sheet surface are described hereinafter.
Firstly, a generally known method is that disclosed in U.S. Pat. Nos. 3,337,431; 3,296,100 and 3,679,554, in which, in the plating process of a steel sheet, two layers comprising a metallic chromium layer and a hydrated chromium oxide layer of an appropriate thickness are deposited on the said steel sheet surface. In this method, however, it is difficult to control the amount of hydrated chromium oxide deposited on the steel sheet surface, and a close control of bath composition is required.
Another generally known method is one disclosed in French Pat. No. 7,125,682, comprising depositing a metallic chromium layer and a hydrated chromium oxide layer on a steel sheet surface in the plating process, and then chemically treating the thus deposited film bearing steel sheet surface. In this method, properties such as corrosion resistance, paint adhesion and adhesive property of the steel sheet surface are improved, whereas the said chemical treatment process not only impairs acceleration of the production line speed, but also exerts adverse effects on the surface exterior view of the sheet surface.
Further, as disclosed in British Pat. No. 1,187,785, a method comprising causing deposition of metallic chromium on a steel sheet surface in a plating process, and then forming a hydrated chromium oxide layer by a chemical conversion treatment of the said deposited surface, is known. It is very difficult, however, to impart satisfactory properties to the steel sheet surface by a simple cathodic electrolytic treatment in such a bath containing hexavalent chromium.
In addition, in the treating methods as mentioned above, it is impossible to fill with hydrated chromium oxide, pores and other defects present in the metallic chromium layer formed on the steel sheet surface, since the cathodic electrolytic treatment is applied in an electrolyte containing hexavalent chromium, and this makes it impossible to impart a satisfactory corrosion resistance to the steel sheet surface.
With these facts in view, there has been a desire to obtain a steel sheet having a bright appearance and a high corrosion resistance by making up pores and other defects present in the chromium plating layer of an electrolytic chromate treated steel sheet or a metallic chromium plated steel sheet. A steel sheet having such surface properties has not as yet been proposed.